European Patent Specification Publication No. 88564 describes a mains telecontrol system in which each transmitted "1" bit is represented by a first pseudo-random sequence of 1024 bit-elements (referred to as "chips"), and each "0" bit by a second such sequence. In the receiver, each sequence of 1024 chips is correlated with two reference sequences stored in the receiver to determine whether the received sequence represents a "1" or a "0". Since the transmitter and receiver are not synchronised, the correlation must be repeated for each phase; in order to expedite the operation, the correlation is performed for eight different phase locations simultaneously. Once the 1024 bit sequence has been received, the receiver contains 16 correlation values each consisting of a number stored in a "bin". To process the correlation values each of them is squared and then the resultant products in each set are added, the two sums being compared in order to determine whether the received sequence represents a "1" or a "0" .